1. Field of the Invention
This invention relates to a system for creating images or tracking objects using a plurality of sensors.
2. Description of the Prior Art
Video images are frequently created to show a scene or record events occurring at a selected location over time. These images are frequently displayed on a video monitor wherein the image is comprised of a set of pixels. Each pixel has either an assigned grey level value or color. These images are created from signals emitted from sensors which are positioned to observe a scene. These sensors could be anything from video cameras to infrared detectors. Typically, the sensors produce analog signals corresponding to the scene which they observe. The analog signals are digitized to create digital images of the scene. Those digital images can be stored in a memory, displayed on a screen or directed to a variety of processing systems which may either extract information from the image or modify the image in accordance with predetermined procedures or reconstructed for display on a screen.
In many applications the sensor or optics associated with the sensor are moved to enable the sensor to survey across a desired field of regard. Such systems generate data in serial form as the sensor or the optics moves across the field of regard. The scan rate associated with such movement determines how rapidly an image can be created and can also affect the clarity of the image which is created. At the present time the data rate available from serially scanning systems is much lower than the ability of processors to handle the generated data. Hence, image processing capabilities have evolved which can process image data much faster than image data can be generated by the serially scanning systems.
When serial scanning is used, images are revisited and updated at a relatively low rate. These low update rates limit passive ranging performance as well as clutter discrimination. Serially scanned systems also have necessarily shorter detector dwell times which limit the sensitivity and detection range. The combined degradations in ranging, discrimination and detection can be severely limiting in tactical and surveillance applications.
Another problem associated with serial scanned systems is that they generally use gimballed optics and serial scan mechanisms which have moving parts that are costly and unreliable. Large protruding apertures are required to maximize coverage. For protection such apertures require spherical windows or greenhouse flat windows. Such spherical windows are generally not desired in aircraft because they make the aircraft easier to detect and also interrupt the aerodynamics of the airplane body. Gimballed optics are not generally compatible with conformal mounting if a large field of view is required. Costly assembly, alignment and maintenance is also characteristic of gimballed optics. Hence, there is a need for an image and tracking system which can cover a wide field of regard without using gimballed optics or other moving parts.
The art has suggested that serial gimballed sensor scanners be replaced with a large number of staring sensors fixed to a host platform. Staring focal plan arrays (FPA's) are distributed to obtain maximal coverage with minimal moving parts. Each array generates a series of signals corresponding to the field-of view which it observes. Those signals must normally be combined with signals from other arrays to generate a full field-of-regard. Since each array stares at its field-of-view, the processing time to create such images can be 1,000 times longer than is available with serial scan systems. Another problem with using parallel arrays is that platform motion can be crippling to long staring time sensors that are mounted on the platform. For example, a 100.degree. per second motion would move scene data across 116 pixels if each sensor's instantaneous field-of view is 150 .mu.r and the integration time is 0.01 seconds. Vibration levels can reach 10 pixels, peak to peak on common aircraft platforms. The resulting loss of signal to noise ratio as well as spatial information is a major problem for staring focal plane arrays.
In the majority of cases, the multiplicity of sensors required to achieve distributed aperture systems requires a greater processor load and increased complexity. Data is available from all distributed aperture scenes simultaneously for each frame. Previously, the parallel processing load has been considered unmanageable when scaled from conventional systems. For example, a distributed aperture concept can easily represent a 30-million pixel processing requirement. These pixels receive support on a parallel frame-to-frame basis. Superficially, multiple sensors appear more costly and complex as well.
Intra-sensor alignment is important for tracking and clutter discrimination. Relating data to a common inertial reference frame is also important for "pointing" targeting systems accurately. For multiple sensors sensor-to-sensor alignment is important. Sensors must be maintained in relative alignment to one another or the signals from them must be corrected to account for any misalignment which occurs.
Consequently, there is a need for an image and tracking system which can generate images from sensors subjected to motion and vibration. The sensors should either be positioned or movable to cover a wide field-of-regard. The signals from the sensors must be integrated and normalized in a relatively short time to account for overlaps in fields of view among the sensors and movement of sensors resulting from vibration or movement of the platform on which the sensors have been mounted.